We made relationships with many different photographers who claim to refer us. I never heard of a referral fee though. I probably wouldn't pay it, but offer to refer them in return.
Probably 50% is through word of mouth, the other is through our website. We set up great Search Engine Optimization so when you search for my city and "wedding videography" we're the first one that comes up.
Bridal shows never worked well for us, we got maybe one wedding out of one we did.
We also just subscribed to Wedding Wire, but I'm not sure if it will do anything for us.